1. Field of the Invention
The present invention relates to a turbomachine frame member for rotatably supporting a turbomachine shaft. More particularly, the present invention relates to a turbomachine frame member that includes an inner annular hub, an outer casing, and a plurality of struts that extend between the hub and the casing, wherein the struts are bolted to the outer casing to provide a lighter, yet sufficiently rigid frame structure.
2. Description of the Related Art
Turbomachines, such as gas turbine engines having rotatable shafts that carry compressors and turbines, or fans and turbines, have their shafts supported in bearings that are housed in support frames. The support frames include inner annular hubs in which the bearings are positioned and outer annular casings that define the outer surface of the engine. The hubs and casings are concentric and are spaced from each other in a radial direction to define an annular flow passageway.
Between the inner hub and the outer casing are a plurality of substantially radially-extending, circumferentially-spaced members that interconnect the hub and the casing. When securely connected together, the components provide a rigid supporting frame for rotatably supporting a drive shaft and also for defining the annular flow passageway. The radial members have exterior surfaces that are generally airfoil-shaped in cross section, with the chords of the airfoil shapes extending in a generally axial direction of the support frame to minimize flow interference.
When utilized in relatively cool sections of a gas turbine engine, such as in the compressor section, the support frames can be cast as an integral structure, or they can be fabricated from cast or sheet metal components that are welded or otherwise joined together to provide a rigid frame. However, in hotter sections of a gas turbine engine, such as downstream of the combustor, in which the frames support a turbine drive shaft, cooling air is generally provided to the interior of the radial members to minimize thermal expansion. The radial members in turbine section frames are often defined by elongated structural struts that are bolted to one or both of the inner hub and the outer casing, and that have through-passageways to allow the flow of cooling air around or through the struts. When such structural struts are utilized, airfoil-shaped outer enclosures or fairings can be provided around the structural struts for aerodynamic efficiency.
When structural struts are bolted either to the outer casing or to the inner hub, or to both in some turbine frame structures, the strut ends can be bolted to a clevis arrangement. The clevis arrangement can be secured to the outer casing or to the inner hub by bolts or by welding. In such bolted-frame structures it is not unusual to bolt a clevis to the inner hub or to the outer casing with four connecting bolts, and the strut end can be bolted to the clevis with two additional connecting bolts for rigidity of the strut-to-clevis connection. Other structural arrangements, in which the strut includes an end cap that is bolted to the strut end and in which the end cap is, in turn, bolted to the inner hub or to the outer casing, can involve the use of as many as eight connecting bolts. The use of a large number of connecting bolts to assemble the components of a turbomachine frame member increases frame assembly and disassembly time, and it also adds considerable weight to the overall frame structure. There is thus a need for a turbomachine support frame structure that provides the necessary strength and rigidity in the operating environment to which the frame is subjected, while minimizing the overall weight of the frame structure.